With recent development of information and communications technology, various wireless communications technology has been developed. Among them, a wireless local area network (WLAN) is technology that allows portable user equipment such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc. in a home, an enterprise or a certain service providing zone to have wireless access to high-speed Internet on the basis of radio frequency technology.
It is presupposed that communications in the WLAN based on institute of electrical and electronics engineers (IEEE) 802.11 standards are performed within a zone called a basic service set (BSS). The BSS zone has a somewhat indefinite boundary since it may vary depending on propagating characteristics of a wireless medium. Such a BSS is basically divided into two configurations of an independent BSS (IBSS) and an infrastructure BSS. The former indicates a BSS that forms a self-contained network and does not allows access to a distribution system (DS), and the latter indicates a BSS that includes one or more access points (AP), a distribution system, etc. and generally employs the AP in all communications including communication between stations.
The station (STA) having desire to access a wireless network may use two scanning methods for searching an accessible wireless network (BSS or IBSS), i.e., a candidate AP or the like.
One is passive scanning, which uses a beacon frame transmitted from the AP (or STA). That is, the STA having desire to access a wireless network periodically receives the beacon frames from the AP or the like managing a relevant BSS (or IBSS), thereby finding the accessible BSS or IBSS.
The other is active scanning. The STA having desire to access the wireless network first transmits a probe request frame. Then, the STA or AP that receives the probe request frame responds with a probe response frame.
There are frequency bands that can be shared by other kinds of wireless communication systems, one of which is a television white space (TV WS). The TV WS is an idle frequency band remained as an analog TV becomes digitalized, and this band corresponds to a spectrum of 54˜698 MHz assigned for TV broadcasting. If a licensed device having priority to use a relevant frequency domain does not use this frequency band, an unlicensed device can use this frequency band.
If the IEEE 802.11 is applied to the TV WS, there is an advantage that coverage is noticeably extended due to the spectrum characteristics of the TV WS. However, in general, if the coverage is extended, the number of STAs within the coverage is definitely increased. As the number of STAs increases, a method for flexibly managing users, i.e., scalability becomes an object of concern. Further, since various wireless communication systems and various unlicensed devices coexist, a problem of co-existence occurs. If distributed coordination function (DCF) and enhanced distributed cannel access (EDCA) protocols of IEEE 802.11 are applied to this environment, the problem of scalability may grow worse.
The DCF is a channel access mechanism used in IEEE 802.11 and based on carrier sense multiple access/collision avoidance (CSMA/CA). Also, the EDCA corresponds to a competition-based medium access method among channel access models proposed in a hybrid coordination function (HCF) provided by extension of a general medium access control protocol. The HCF is a protocol provided by IEEE 802.11e proposed to guarantee quality of service (QoS).
To use the TV WS, an unlicensed device has to get a channel available in a relevant zone on the basis of geo-location database. Also, there is a need for scanning to solve the problem of coexistence between the unlicensed devices using the TV WS.
In the WLAN supporting a TV WS band, the STA performs a scanning procedure for accessing the BSS. Scanning coverage of the STA is limited by transmission power. The transmission power for the scanning in the TV WS is lower than that of after associating with the AP. This is to prevent interference with authenticated equipment in the scanning procedure.
Accordingly, there is a need for a method of solving coverage imbalance due to difference between the transmission power for the scanning and transmission power in a client of the AP.